62 Mr. James Rice on 



walls, each train, as it reaches the wall again, being in part 

 reflected and in part absorbed only to be emitted once more. 

 No we do know something about the molecular movements in a 

 piece of matter at a given temperature, not detailed knowledge 

 concerning any individual molecule to be sure, but knowledge 

 concerning average behaviour. From that knowledge we can 

 infer certain results concerning the movement of the ether in our 

 "constant temperature enclosure" Furthermore, it is possible 

 to test these inferences, and unfortunately while some of them 

 are verified, one very important one is absolutely at variance with 

 the facts. 



To show you where the contradiction arises, I must necessarily 

 try to explain to you Avhat is the kind of knowledge we have 

 about molecular and atomic motion. To do so, I take the 

 simplest instance possible, molecular motion in a body of gas, 

 where the molecules spend the greater part of time practically 

 free from each other's influence or that of the molecules in the 

 sides of the enclosing vessel. The whole theory of such molecular 

 motion has been very thoroughly worked out and can be 

 found in books on the Dynamical Theory of Gases ; the results 

 obtained are numei'ous and in many cases capable of verification ; 

 in fact few single bodies of scientific knowledge can show such a 

 strong chain of influence and verification. One of these results 

 refers to the average energy possessed by any individual molecule 

 of the gas when its temperature is definite. At any instant 

 among the enormous numbers of molecules present — and you 

 must remember we are dealing with numbers of the order 

 30 trillions per cubic centimetre for a gas at atmospheric pressure — 

 as many are travelling within any well-defined limits of direction 

 as within any other similarly defined limits. Otherwise we 

 would be having the gas pressing in one direction more than 

 another. That is, there are no privileged directions of motion. 

 At a given temperature there is one privileged speed ; i.e., there 

 are more molecules with speeds varying by not more than say 

 1 metre per second from this speed than there are molecules 



